In camera optics, it is known that sensitivity becomes lower at a location further away from the center of the imaging plane due to oblique incidence of a principal ray. This is called “shading”. Particularly, if the respective colors of RGB (Red, Green, and Blue) differ in sensitivity decrease rate, white balance is degraded, and colors are generated. This is called “color shading”, for example. Color mixing also occurs, as light reaches adjacent pixels at the same time.
Correcting such shading and color mixing is preferable in improving the performance of an image sensor. In an image sensor in which the RGB photodiodes (PDs) or photoelectric conversion layers are stacked in the vertical direction (vertical spectrum), shading, color shading, color mixing become conspicuous due to oblique incidence. This is because light that enters obliquely does not easily reach the PDs or photoelectric conversion layers on the back side.
PTLs 1 and 2 disclose pupil correction to be performed on the organic photoelectric conversion films or on-chip lenses of the respective pixels. However, it is not possible to avoid shading due to the difficulty of pupil correction on the red (R) and blue (B) PDs, and color mixing caused by light entering adjacent pixels due to oblique incidence in the PDs.
In view of those problems, PTLs 3 and 4 discloses shading correction to be performed by modifying the lens shapes of on-chip lenses, and converting oblique incidence of light into normal incidence. When the lens shapes of on-chip lenses are modified, shading correction sufficient to achieve normal incidence of a principal ray can be performed. However, the shapes of the on-chip lenses of the respective pixels are preferably varied in accordance with the inclination of the principal ray.
As a result, the design cost and the process cost for forming OCLs become higher. In a camera that has a telescopic function, oblique incidence of a principal ray can be corrected at the nearest end of depth of field, but a principal ray enters almost perpendicularly even at an edge of the imaging plane at the furthest end of depth of field, if the shapes of the OCLs are fixed. As a result, degradation in characteristics might occur.
PTLs 5 through 7 disclose shading correction by attaching a transparent material having a convex curved surface. However, any specific structure for that has not been disclosed, and there is a possibility that sufficient correction will not be performed, and further degradation will occur, depending on shapes such as curvatures. With a convex transparent material of a fixed type, oblique incidence of a principal ray can be corrected at the nearest end. However, as long as the shape of the convex curved surface is fixed, the principal ray enters almost perpendicularly even at the edge of the imaging plane at the furthest end of depth of field. As a result, more characteristics degradation might occur.